A review of the use of fibre-optic sensor technologies for humidity sensing is presented. The paper first provides a brief overview on the basic concept of what is meant by humidity and on conventional detection methods. This is followed by an extensive review on the various fibre-optic techniques reported for humidity sensing, covering both intrinsic and extrinsic sensor configurations.

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A fiber-optic-based humidity sensor has been fabricated using a fiber Bragg grating (FBG) coated with a moisturesensitive polymer. The sensing concept exploits the inherent characteristics of the FBG and is based on the strain effect induced in the Bragg grating through the swelling of the polymer coating. A direct indication of the humidity level is given by the shift of the Bragg wavelength caused by the expansion of the sensing material. The FBG sensor used in this work has an approximate coating thickness of 33 m and was exposed to different humidity levels at room temperature. The sensitivity of the sensor was estimated to be about 4.5 pm/%RH at a wavelength of 1535 nm, this being obtained through a process of linear regression. The resulting uncertainty in the measurement is 4%RH and the response time of the sensor and the moisture expansion coefficient of polyimide were obtained from a series of experimental investigations and cross compared with the results of previous work.

A fibre-optic-based humidity sensor has been developed and used for the measurement of moisture absorption in concrete. The sensor was fabricated using a fibre Bragg grating (FBG) coated with a moisture sensitive polymer. To investigate the use of this sensing technique for the detection of moisture ingress in concrete, the sensor was embedded in various concrete samples of different water to cement ratios which were then immersed in a water bath. A direct indication of the humidity level within a sample is given by the shift of the Bragg wavelength caused by the expansion of the humidity-sensitive material coated on the fibre. The sensor itself exploits the inherent characteristics of the FBG, with its operation being based on the strain effect induced in the Bragg grating, through the swelling of the polymer coating.It was found that optical-fibre-based humidity sensors of this type form a basis for determining the changes in the moisture content in different concrete samples, indicating potential new applications of the sensor system to ensure the integrity of civil engineering structures in which they are used.

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A long period grating (LPG)-based optical fiber sensor, coated with polyvinyl alcohol, has been developed for relative humidity (RH) measurement over the range from 33%RH to 97%RH, for which the grating response has been monitored. The fiber optic sensor using the LPG has been designed to monitor the change in resonance loss which is then related to the humidity measured through calibration of the sensor with a series of materials of known refractive index. Results on the calibration and use of the sensor are reported and considered in this work, in light of other gratingbased methods for humidity measurement.

Fiber optic-based humidity sensors, fabricated using fiber Bragg gratings coated with moisture sensitive polymer, are employed in detecting and monitoring the movement of moisture through standardized cubes made from samples of different types of structural concrete. Data obtained can give information on the properties of different types of concrete and thus on the migration of dissolved salts, e.g., sodium chloride, which is important in view of their deleterious effects on reinforcement bars within concrete. To undertake the research, two such optical fiber sensors were placed at varying distances from the edge of the cube face to the inner part of the concrete sample and subjected to a constant head of water at a constant temperature. The optical fiber sensors reacted to the ingress of water by detecting the moisture migrating through the sample, indicated by a shift in the Bragg wavelength of the sensor.

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